The disclosure relates generally to ceramic matrix composites. More particularly, embodiments herein generally describe recession resistant ceramic matrix composites, coatings and related articles and methods used in the gas turbine and aerospace industries.
Higher operating temperatures for gas turbine engines are continuously being sought in order to improve their efficiency. However, as operating temperatures increase, the high temperature durability of the articles of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel, and cobalt-based superalloys. While superalloys have found wide use for articles used throughout gas turbine engines, and especially in the higher temperature sections, alternative lighter-weight substrate materials have been proposed.
Ceramic matrix composites are a class of materials that consist of a reinforcing material surrounded by a ceramic matrix phase, and are currently proposed for use for higher temperature applications. Ceramic matrix composites can decrease the weight, yet maintain the strength and durability, of turbine articles used in higher temperature sections of gas turbine engines, such as airfoils (blades and vanes), combustors, shrouds and other like articles that would benefit from the lighter-weight these materials can offer.
It is well known that one of the critical problems in using silicon carbide ceramics is the loss of thickness of the ceramic matrix composite (“CMC”) resulting from the reaction of the ceramic with the moisture in the combustion gases. Consequently, environmental barrier coatings (“EBCs”) are used to protect the CMCs from the loss of thickness or the recession of the ceramic by volatilization. EBCs developed to date are multi-layer coatings with a bond coat of silicon or silicon-containing material, which on oxidation forms silicon oxide.
Experience to date has shown that environmental barrier coatings usually have local spalls, such as caused by foreign object damage or handling damage. It is believed that for most hot stage components, this would result in very high volatilization rates locally in the region of spalls resulting in the formation of holes in the CMC components. In particular, when EBCs spall off, the underlying substrate is exposed to the moisture-containing combustion gases, and in some other cases (for example, when the EBC is porous or cracked), the moisture can diffuse through the porous/cracked layer to oxidize the underlying substrate and cause recession of the substrate. This is believed to be one of the major problems in the commercialization of CMCs, and the ceramic community has been working to solve this problem. It is, therefore, desirable to increase the recession resistance of the CMC substrate. It is also desirable to increase the robustness of the EBC system so that when the local EBC spallation occurs the recession resistance of the system is still acceptable.
Moreover, there is a strong driving force to develop ceramic matrix composites for applications at temperatures up to 2700 F. Volatilization of silicon as silicon hydroxide is one of the key problems with such composites because it leads to loss of thickness with time. Environmental Barrier Coatings (EBCs) are used to alleviate this problem. However, many EBCs use a silicon bond layer on the surface of the CMCs, and silicon melts at temperatures around 2550 F. Therefore, silicon-based coatings are currently not practical at temperatures over about 2550 F. Therefore, not only is there a need in the art for recession-resistant CMCs, there is also a need in the art for new EBCs that can operate at higher temperatures. There is also a need for robust EBCs so that the recession of the ceramic substrate is acceptable even when there is local spallation of the EBC layers. In short, there is a need in the art for improved recession resistant CMCs, EBCs, articles and methods for making them.